Method for improving paper and board&#39;s resistance to the penetration of liquids

ABSTRACT

This invention brings a new aspect in papermaking by spraying hydrophobic chemical directly onto the paper surface in the dryer section of the paper machine, as a two-step process where high solids starch is sprayed on the paper to seal the sheet&#39;s surface and thereafter the hydrophobic chemical is sprayed on the paper surface. This invention is a completely new method of hydrophobic sizing, spraying a small amount of a suitable hydrophobic chemical on the moving web in the dryer section, instead of adding hydrophobic chemical in the wet-end of the paper machine, or mixing it in with starch at a size press. This invention reduces the amount of hydrophobic chemical used compared to any of the other processes, it simplifies wet-end chemistry, and it puts the hydrophobic sizing exactly where it is needed, on the paper&#39;s surface, which in many cases will be a single side of the paper.

FIELD OF THE INVENTION

This invention relates to a new method of applying hydrophobic sizing directly on the moving web in the drying section of the paper machine for the production of paper, board, and packaging grades, where water repellency and the resistance to water penetration are critically important.

BACKGROUND TO THE INVENTION Sizing Concepts in the Paper Industry

As is well known in the paper industry and with users of paper products, most grades of paper and board, excluding those used for tissues and hygiene applications, need to be resistant to wetting and the penetration by liquids.

With reference to the pulp and paper industry's general views and contained in the papermaking handbook, Paper Making Science and Technology, published by The Finnish Paper Engineer Association, “internal sizing” is typically defined as a process wherein chemical additives are introduced to make the surface more water-repellent. The internal sizing process traditionally includes the stages when chemicals are added to the papermaking furnish and retained on the fibers by appropriate means in the wet end system of the paper manufacturing process. With the traditional “internal sizing”, the main purpose is to modify the surface of the paper to control water penetration, in order to either limit the water penetration into the sheet in conventional sizing or coating operations.

Here one can note the term sizing. In paper making, normally sizing is known as “surface sizing”, when starch is applied on the surface of the paper web in the dryer section of the papermaking process. As is well known in the science of papermaking, starch has a positive impact on paper strength, stiffness and surface properties. The purpose of this sizing is to improve surface strength, improve internal bonding and/or reduce the openness of the surface. In newsprint or book papers printed in cold-set presses, starch is added to improve surface strength and thereby reduce linting or dusting. The same applies to other offset printing papers and to office papers used for copying. In fine papers, starch is used to increase internal bonding, to improve bulk and stiffness and to avoid de-lamination of coatings and paper layers. In packaging grades starch is used to improve internal strength and stiffness, also to decrease tendencies for de-lamination of multi-layered grades.

To differentiate between the different types of sizing, the inventors here are using the terms “hydrophobic sizing” for the process where a hydrophobic agent is added to improve the paper's resistance to liquid penetration and the term “surface strength sizing” is used for the well known process of adding starch onto the web surface to improve both surface and internal strength and to reduce the openness of the surface.

Today, most typical and traditional “internal sizing” methods are based on synthetic cellulose-reactive systems based on either alkyl ketene dimers (AKD) or alkenyl succinic acid derivatives (ASA). The following gives an overview of the complications with the existing hydrophobic sizing methods of today to improve the paper surface's water repellency. This overview serves as the background for this invention.

ASA

Normally ASA is a yellowish oily product. This 100% active substance can be stored as such but must be well protected from water or humidity. It will not dissolve in water and, prior to application in papermaking, must be emulsified on-site at the paper mill. A small amount (3%-6%) of activator is added, plus cationic starch and a synthetic cationic polymer which serve as stabilizers. Activators are surface-active agents that promote effective emulsification. Emulsion stability is best with synthetic polymer, and sizing efficiency is best with cationic starch. The efficiency improves with the amount of starch used. The ratio of starch to ASA is usually in the range 2:1 to 4:1.

Good solids retention is extremely important in ASA sizing, and ASA is a very effective size if applied in a correct way. A relatively small dosage (usually 0.1-0.18% based on the total furnish solids) is required to produce the desired degree of sizing. As with any process of internal sizing, the fines and filler fractions with a large surface area will consume a large proportion of the amount of size introduced, and ASA adsorbed onto the unretained fines and filler surfaces will pass into the white water. Poor retention thus results in an ASA excess on the fine particles circulating in the paper machine short circulation system. This causes poor sizing results and leads to a build-up of deposits on the wet end and in the press section, which can cause paper machine runnability problems. Therefore the selection of retention aid is a critical issue, because the excess amounts of ASA in the system cause poor retention, contamination in the press felts, and press-roll picking.

Another issue is the emulsification of the ASA has to be carried out in balance with the paper machine's operation. Whenever breaks occur or interruptions on the paper machine are expected to last longer than a few minutes, a continuously operated ASA unit must be stopped, emptied, and restarted later. ASA fed during downtime will return to the hydrolyzed state and aggravate the situation.

The great advantage in favor of ASA is the high rate of cure in comparison with AKD size. The reaction with cellulose hydroxyls takes place rapidly in the dryer section of the paper machine at less than 5% web humidity. In most cases more than 90% of the attainable hydrophobic sizing potential is achieved before reaching a size press unit in the dryer section.

AKD

AKD is another chemical used for “internal sizing”. Structurally, alkyl ketene dimers (AKD) are unsaturated lactones.

As is the case with many wet end additives, for each dosage into the papermaking furnish, the wax must be converted into tiny particles dispersed in water. Since the first step is carried out with molten wax, it is correctly described as emulsification.

The effect of AKD in the paper machine system is the following:

The dispersed, cationically stabilized particles are first adsorbed onto the fiber surface by electrostatic attraction. The AKD-dosage point for carrying out this step varies and lies typically within the machine chest fan pump level box. Additional cationic starch can be added to assist the retention of AKD either before or along with the dosage of the AKD.

As the web is being heated and dried in the paper machine, the adsorbed AKD wax starts to melt and after spreading it covers at least part of the fiber surface as a thin layer.

The chemical reaction with the cellulose hydroxyls develops to a sufficient level once the water has been evaporated from the web and the sheet moisture is 3%-5%. The sizing effect is still improving once the reel has been produced on the machine.

In addition to the web moisture content, the sizing effect also depends upon pH and temperature.

Fillers and fines increase the amount of AKD required in much the same way as is the case with the size demand in ASA sizing. Two significant factors decrease sizing efficiency in the wet end of the paper machine. The first is that fillers and fines are not retained as efficiently as fiber, and thus a large proportion (more than 50% on most paper machines) of the material carrying adsorbed AKD ends up in the white water. The second factor is that the retention system can agglomerate the filler with adsorbed AKD so extensively that a large proportion of the size is trapped inside the agglomerates and cannot spread over fibers and react in the normal way.

Internal hydrophobic sizing creates paper machine wet-end complications. The most important factor with both AKD and ASA sizing is to have an optimized overall retention system. With proper retention and flocculation of fillers and fines, the total surface area of particles re-circulated is reduced; these would otherwise consume much of the sizing agents like AKD or ASA, cationic starch, and promoter added. Optimization of the overall retention system is a critical issue and should include mapping of the anionic and cationic levels of the wet end, and of inorganic and organic fines in the short circulation loop. Nevertheless, it is well known in the industry that even with an optimized retention system, a significant amount of the hydrophobic chemicals is not retained and travels with the white water into the short circulation system of the paper machine. It is generally estimated that 40-50% of the chemicals are not retained.

Effects of Hydrophobic Sizing

Normal, or good sizing, is the ability to achieve a sizing level where the paper at the reel is within the desired specifications and to maintain this level over time. Size efficiency is typically defined as the relationship between the size response on the reel and the amount of size added to the papermaking system. Poor sizing results in paper out of specification at the reel. Size reversion is the phenomenon when paper is in specification at the reel but, upon aging, the sizing level decreases to a lower level, and then remains constant. The paper might then be within or outside the specifications.

There are numerous tests to measure the effect of hydrophobic sizing and the paper or board surface's resistance to water and liquid. The Cobb test is a well proven practical measurement which correlates well with changes in the sizing effect. Cobb measures the amount of water absorbed and is reported as g/m2. The test is carried out by exposing the surface of the test sample to water, for a specified time, usually from 30 seconds to 10 minutes, depending on the grade and the degree of sizing. For the majority of paper grades, the test time is 60 seconds and the circular area of the sample 100 cm2. The 60 second Cobb-values normally ranges from 25 to 50 g/m2. However, the test is inexact when used to measure either hard-sized or very slightly sized paper or board, or with very thin or absorbent paper.

The information above submits that the traditional “internal sizing” to improve paper surface water repellency and resistance to water penetration is a complicated process. It appears logical to view this from a functional perspective; if the paper surface's water repellency and resistance to water penetration is to be improved, the hydrophobic chemical should be applied on the paper's surface, where the chemical can fulfill its intended purpose. It is the inventors' view that chemicals with the specific purpose to improve paper surface characteristics should not be added in the wet-end where the chemical mixes into the web structure and the effect is diluted, but rather deposited directly onto the paper's surface. Adding chemicals in the wet-end also leads to the situation where the chemical retention is poor and much of it is transported with the process water into the paper machine's short circulation system. It is estimated that the retention of chemicals on the wire section, for example wet-end starch, is in the range of 50-60%.

With this invention, the perspective is to improve the water repellency and resistance to water penetration of the end-product. The end-product being defined as the base paper or board for coating or converting or as the final product the ultimate consumer uses. It is essential to the paper manufacturer that end-use requirements are met, and that the degree of sizing of the product remains as constant as possible until the ultimate consumer uses it. This invention integrates the view that with a new high solids spray-sizing concept being introduced in the industry that a new hydrophobic spray sizing process can be developed to follow as a second step.

The spraying of starch on the paper surface, followed by a set of rolls forming a nip [See FIG. 1] has been introduced to the industry through the European patent EP 0 682 571 B1 and by a Finnish patent, “Patentti no 96894”. Both patents were developed by Patrick Sundholm, who is one of the inventors of this invention. The spray is created by a nozzle [#1 in FIG. 1], called an air nozzle, where air, steam and the starch mixture creates a fog of starch which is then carried to the paper surface. It is followed by two rolls [#2 in FIG. 1] creating a nip [#3 in FIG. 1] to force the penetration of the starch into the paper. The implementation of this process has shown that the starch can be added at a high solids content above 20%, even up to 30-35%. The amount of starch added is approximately 1 to 5% of the paper's total weight.

The first industrial application has been installed in 2010 after completing several test runs on a pilot scale at a research facility in Imatra, Finland. Patrick Sundholm was a key member overseeing these trials at the research facility.

The new starch application system opens a new window in paper making and creates the base for the invention described in this paper. With the high solid starch spray, and the following set of two rolls forming a nip, the operating window for controlling the penetration into the web's surface and thus the surface “closeness” reaches new dimensions. The surface “closeness” correlates with air permeability, measured by Gurley. Several trials were carried out to analyze the air permeability and the “closeness” of the sheet surface, as a function of the amount of starch and the nip load. It was found that the starch penetration can be well controlled. A situation, where the starch is only slightly penetrated into the sheet and seals the surface can be generated. One example of a trial is demonstrated in FIG. 2, which charts the air permeability. In FIG. 2, item #1 is the Vertical axis, which is time in seconds and item #2 is the Horizontal axis showing grams per side of starch. Item #3 is the trend line which relates to the triangle data points which depicts zero nip loading and item #4 is the trend line relating to the diamond data points which presents the values with a 40 kN nip loading].

The high Gurley value represents situations where the surface is sealed. The sealing of the surface with starch has a vital function to form a layer on the surface in situations when the objective is to apply and maintain a small amount of a specific chemical directly on the paper surface where the chemical fulfills an essential function, for example, in this case improving the paper or board's resistance to water penetration and improving water repellency. The paper surface is normally “rough and open” when sized with traditional methods, and therefore when a small amount of chemical is added the chemical sinks “between the hills and valleys” on the paper's surface. That effect has been seen when a very thin layer of pigments, 1-2 g/m2 was sprayed in very small droplets. The high solids starch spray sizing is a tool for developing paper making further by better sealing the surface.

The traditional manufacturing concept for most paper grades includes both “internal sizing” to improve paper surface resistance to water penetration, and “surface sizing” with starch to improve the paper or paperboard with both internal and surface strength. In the paper making of today, the normal sequence is that the “internal sizing” in the wet-end is followed by “surface sizing” with starch in the drying section.

It is the inventors' view that the traditional sequence does not appear right, as water repellency chemicals, applied at the very small amount of 0.1% of the paper mass per area unit, should be applied on the very surface to create the best possible effect. Therefore, a better method is to apply the hydrophobic chemical for water repellency and resistance to water penetration in the drying section.

In the industry, it is also known that hydrophobic sizing can be carried out by combining it with the “surface strength sizing” process in the dryer section. In such cases, the ASA for example, is mixed into the starch and transferred to the surface of the paper web with a film sizer or a pond sizer. The disadvantage of this solution are twofold; the negative impact which ASA or AKD has on starch properties, plus the diluted effect of the ASA or AKD as the chemical is driven with the low consistency starch/water mixture into the inner layers of the paper by the nip pressure given that the solids content of the starch is low.

Other examples in the paper industry can be found where various hydrophobic agents are added into the starch to attempt to improve paper surface water repellency ability. However, this is the only invention where the hydrophobic chemical is imparted and remains on the surface where it is best needed to repel water.

It is the inventors' belief that the hydrophobic sizing process is best done by a two-step process, by first treating the paper with a high solids starch spray to achieve the best possible internal strength, surface strength and with the special purpose of sealing the surface of the paper in the first step, and thereafter by spraying the hydrophobic chemicals as a second step. This better accomplishes the specific improvement of water repellency and resistance to water penetration of the paper or board's surface. The two step process of spraying of the said chemicals affords the opportunity to control the exact amount required to create a uniform layer on the surface to improve the water repellency and water penetration resistance to an exact and desired level.

The two-step process by first adding high solids starch by spraying and adjusting the starch penetration with a set of two rolls forming a nip to seal the sheet surface and thereafter adding a hydrophobic chemical by spraying to reach the best possible effect of the chemical for improvement in the paper or board's surface to repel water and resist water penetration. This two-step process of hydrophobic sizing, according to the inventors' view, has not been described in any industry literature and is not installed on any production line for manufacturing paper or board.

BRIEF SUMMARY OF THE INVENTION

This invention is a completely new method of hydrophobic sizing. It is a two-stage process. The first stage is spraying a high solids starch solution directly onto the surface of the paper or board in the drying section of the paper machine and then adjusting the starch penetration into the sheet with a set of two rolls forming a nip and sealing the sheet's surface. The second stage is spraying a small amount of a suitable hydrophobic chemical directly onto the sealed surface of the sheet in the after drying section of the paper machine to reach the best possible effect of the chemical for improvement in the paper or board's surface to repel water and resist water penetration. This process forms a true invention in paper making.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing of the spray starch sizing station.

FIG. 2 is a chart presenting the air permeability achieved during trail runs on a pilot unit.

FIG. 3 is a side view drawing of the spray box which is the integral part of each spray station.

FIG. 4 is a cross sectional drawing of the air nozzle which is in the spray box.

DESCRIPTION OF THE INVENTION

This invention brings a new aspect in paper making by simplifying the wet-end chemistry and presents that the hydrophobic chemical is added directly on to the moving paper web surface in the dryer section of the paper machine rather than in the wet end and the inherent problems resulting from that addition point. It is also an improvement to the other industry practice of mixing the hydrophobic chemical with starch and applying it with a size press. This invention reduces the amount of hydrophobic chemical used compared to any of the other processes and affords the paper makers with the ability to put the hydrophobic sizing exactly where it is needed on the paper's surface, which in some cases will only be on a single side of the paper.

The fundamental foundation of this invention is the spraying of hydrophobic chemical directly on to the paper surface in the dryer section, as a two-step spraying process; first with high solids starch being sprayed on the paper surface, followed by a set of two rolls forming a nip to allow for controlled penetration of the starch into the paper, and then second, with the spraying of the hydrophobic chemical for improving the water repellency and the resistance to water penetration directly on to the moving web in the drying section, after the starch sizing stage and at a stage where the web has reached a sufficient dryness of +80%.

Thus this invention can be described as follows:

In the production of magazine paper, graphical paper grades, folding box board, or container packaging grades, where the water repellency and water penetration resistance characteristics of the paper surface play an important role, the hydrophobic sizing is achieved by spraying a small amount of a suitable hydrophobic chemical, such as ASA or AKD, on the moving web in the drying section of the machine, instead of adding the chemical in the wet-end of the paper machine.

The sequence of sizing to be changed from the traditional method of “internal sizing” in the wet-end and “surface sizing” in the dry-end to a new two-step process of spraying high solids starch “strength sizing” in the dryer section, followed with “hydrophobic sizing” by spraying the hydrophobic chemical directly onto the surface of the paper web in the dryer section.

The hydrophobic spray system for improving the water repellency and resistance to water penetration is positioned after the starch spray station in the dryer section, where high solids starch is sprayed on the paper surface, followed by a set of two rolls forming a nip. Before adding the hydrophobic chemical to improve the water repellency, the paper web dryness has to reach a point where the moisture content is 20% or less.

The invention is based on known spray technology, which has been around in the industry for some 50 years and which, as mentioned, has been modified for surface sizing with starch in the following patents; EP 0 682 571 B1 and “Patentti no 96894”.

The technology forming the foundation for this invention is described as follows:

A spray box, where there are one or multiple rows of nozzles, spaced in the cross directions on a distance of between 3-10 cm, and if there is more than one row, the distance between the rows is 5-10 cm. FIG. 3 depicts dual spray boxes with two rows of air nozzles for spraying both sides of the sheet [#1 in FIG. 3 is the moving paper web, #2 and #2 a are the spray boxes containing the air nozzles and #3 and #3 a are the air nozzles], which would be the arrangement in a high solids starch application. Hydrophobic sizing could be either a single or dual box design depending on whether water repellency is desired on one or both sides of the paper, respectively.

The spray is created in each nozzle by directing a “gas under pressure” to the nozzle tip, where the chemical is broken up into small droplets. This technology has been described in several patent applications, including U.S. Pat. No. 4,944,960 where Patrick Sundholm was an inventor. The nozzles are called “air nozzles” or “low pressure nozzles”, see FIG. 4 [#1 is the channel for the starch or the chemical addition, #2 and #2 a is are the channels for the gas under pressure, and #3 is the nozzle opening, which varies from 1 to 6 mm].

The benefits of the invention are as follows:

The consumption of chemicals will be drastically reduced as the hydrophobic chemical is added on the surface where the affect is needed to bring about the maximum benefit.

No cationic starch is required to be fed into the wet-end to improve sizing efficiency.

It is foreseen that the paper machine's runnability will drastically improve, when the negative effects of ASA and AKD in combination with cationic starch in the wet-end systems are eliminated. There will be reduced contamination of the fines in the paper machine's short circulation system, which will also have a positive impact on paper machine runnability.

It provides greater flexibility in selecting retention aids because it simplifies wet-end chemistry with the removal of ASA or AKD in combination with cationic starch at the wet end of the paper machine.

The costs of hydrophobic sizing will be reduced as the retention of the chemicals is 100% since the chemical is sprayed directly on the paper surface. This means that no amounts of AKD and ASA will re-circulate in the paper machine systems with all the negative effects as described earlier in this document.

Stability in sizing and less waste is created as no chemicals to improve water repellency or resistance to water penetration are released into the system during situations when a paper machine web break or operating interruption occurs.

Controlling the exact dosage according to the specified target for the effect on water repellency is another significant positive attribute.

The consumption and cost of hydrophobic sizing will be significantly reduced because it will only be on the paper's surface where it is needed and on many grades of paper that is only on a single side of the paper.

The items presented herein represent significant improvements in the science of paper making. 

1. A method to improve the surface of paper or board to repel water and to resist water penetration into the sheet by applying a hydrophobic chemical directly on the paper surface by spraying the chemical on to the moving paper web in the drying section of the paper machine.
 2. A method according to claim 1 where the hydrophobic chemicals used for sizing are ASA, AKD or any other similar chemical which increases paper surface water repellency and resistance to water penetration.
 3. A method according to claim 2 where the spraying is done with an application air nozzle where the gas, or air, or steam or a mix are pressurized and are the force for breaking up the hydrophobic chemical into small droplets forming a spray and carrying the hydrophobic chemical onto the paper web surface.
 4. A method to improve the surface of paper or board to repel water and to resist water penetration into the sheet by applying a hydrophobic chemical on the paper surface by spraying the chemical on to the moving paper web in the drying section of the paper machine, which is installed after a spray sizing system for starch which has a set of two rolls forming a nip to control the penetration of starch into the web surface.
 5. A method to improve the surface of paper or board to repel water and to resist water penetration into the sheet by a two-step integrated process for both paper and board where internal strength and surface strength properties are improved by spraying high solids starch at a solids content greater than 15% onto the moving web in the drying section followed by a set of two rolls forming a nip to control the penetration of starch into the web surface, thence followed by the spraying of hydrophobic sizing chemicals onto the moving web in the drying section to improve the paper surface's ability to repel water and to resist water penetration into the sheet by using a spraying device containing of a set of pressurized air nozzles arranged over the cross section of the sheet and in the machine direction in one or more rows. 